1. Field of the Invention
The present invention relates to methods for fabricating insulated, concentric tubular bodies and the bodies fabricated by such methods. Insulated tubulars or pipe of this type are sometimes referred to as xe2x80x9cpipe-in-pipexe2x80x9d or xe2x80x9ctube-in-tubexe2x80x9d fluid conduits. Insulated, concentric tubular fluid conduits are employed in well completions, pipelines, refineries, and other processing installations.
2. Prior Art Setting of the Invention
The tube-in-tube insulated pipe employed in oil and gas wells is typically joined together end-to-end by threaded connections. In the case of some of the designs employed in oil and gas wells, the threads must be formed on the tubing after the concentric pipe bodies have been assembled. The typical insulated section employs a threaded external tube surrounding a shorter, smaller diameter tube that is welded into the larger tube. The assembly may also be heat treated after the inner tube is installed. The threads are preferably formed on the outer tube after the inner tube is installed and the assembly is heat treated.
Conventional threading operations require that the pipe be rotated as threads are being cut into the end of the pipe. Threading speeds of 150 rpm or more are common. Relative movement between the inner and outer pipe bodies as the assembly is rotated can cause harsh vibration and other undesired mechanical movement that interferes with the threading process.
The relative radial position of the inner and outer tubes can be fixed by employing multiple rigid spacers in the annulus between the two tubes. One commonly employed technique is to provide a hinged metal stamping with raised ears to close the gap between the inner and outer tubes. Because of the use of sheet stock as the spacer material, the contacting ears form a flat surface engaging the inside surface of the surrounding tube.
Another technique employed to fix the inner and outer tubes is to employ a square stock that has been cut to short lengths and welded onto the outside surface of the inner pipe. Because the spacer is cut from square stock, the contact ends form a flat surface contacting the inside surface of the inner pipe.
In both of the described prior art spacing techniques, the design and volume of these spacers can significantly increase the heat transfer between the inner and outer conductors. Benefits of a more stable assembly for threading purposes are offset by the increases in heat transfer between the tubes.
The shape, size, and distribution of the centralizers also affect the assembly process of inserting the inner tube into the outer tube. Multilayer wrappings of insulating material are often disposed over the smaller diameter tube between the axially spaced centralizers. It is typical to insert the inner tube, equipped with centralizers and insulation material, into the larger tube while the inner and outer tubes are horizontally oriented. As this is done, the inner tube slides along the internal bottom wall of the outertube. This sliding movement can damage the centralizers and the insulation material. Horizontal insertion may also be made difficult when the internal surface of the outer tubular body is coarse or irregular such that the centralizers and insulation are abraded or hang up on the surface irregularities.
Many of the problems associated with horizontal assembly of the inner and outer tubes may be avoided by orienting the two tubes vertically while the inner tube is inserted. While the vertical orientation technique can eliminate the described problems associated with dragging the inner tube along the internal bottom wall of the outer tube, vertical orientation introduces other assembly problems. In this regard, the relative axial position of the inner and outer tubular bodies must be closely controlled such that the mid-length of each conduit is coincident. Additionally, the inner tubular body is usually shorter than the surrounding outer body so that the lengthwise centering must be accomplished within the surrounding pipe.
The desired spacing could be achieved by welding temporary lugs or retaining members to the inner or outer pipe at appropriate locations to form a stop that would fix the inner pipe lengthwise within the outer pipe. Such a procedure would require an additional welding step and also introduces an additional point for the introduction of measurement errors.
Centralizing rings having specially configured contact surfaces are affixed to the external surface of the inner tube forming the concentric insulated tubular assembly. The centralizers are provided with a curving cross-section along their radially outermost contact surface to make circumferential line contact with the internal surface of the overlying tubular body to minimize heat transfer between the tubes.
Recesses are formed circumferentially about the external diameter of the centralizing rings to minimize the total contact area between the rings and the surrounding tubing. The centralizing rings are disposed circumferentially about the inner tube such that the recesses in adjacent centralizing rings are offset circumferentially to preclude axially extending, aligned contact surfaces that enhance heat transfer.
The centralizing rings provide a minimum amount of surface contact with the overlying tube while rigidly occupying the annular space formed between the inner and outer tubular bodies to prevent any relative radial displacement between the bodies. The firm line contact engagement provided by the centralizing rings minimizes the heat transfer between the inner and outer tubes and also permits the concentric tubular assembly to be rotated at high speeds during the threading process without experiencing excessive vibration or other undesired mechanical movement.
The damage normally occurring when insulating tubes are assembled horizontally is avoided with the use of a lifting device of the present invention that permits the inner and outer tubes to be assembled vertically in a recessed assembly well. the lifting device permits the inner tube to be raised vertically and aligned concentrically with the vertically disposed outer tube. The lifting device employed to lower the centralizer-equipped, insulation-wrapped inner tube vertically into the outer tube automatically centers the inner tube within the surrounding outer tube. Springloaded legs carried by the lifting device automatically spring radially outwardly once they clear the bottom of the larger diameter tubing. An upward pull exerted on the lifting device brings detents in the legs against the base of the outer tube so that the outer tube is supported by the lifting device and the inner and outer tubes are precisely positioned axially. The combined assembly may then be raised vertically from the assembly well and returned to horizontal position for subsequent working.
From the foregoing, it will be appreciated that an important object of the present invention is to provide a method and apparatus for assembling concentric tubular bodies whereby the bodies are rigidly affixed radially relative to each other to prevent vibration or other undesired motion as the bodies are rotated during the threading of the ends of the tubular bodies.
Yet another object of the present invention is to provide a centralizer ring that can be positioned in the annular space between concentric tubulars to securely fix the tubulars radially relative to each other while minimizing the amount of heat transfer between the tubulars.
Another object of the present invention is to provide a centralizing ring disposed in the annulus between two concentric tubular bodies wherein the centralizing ring forms an interrupted line contact with the surrounding tubular body to minimize the heat transfer between the two tubular bodies.
A related object of the present invention is to provide a curving surface on the external circumferential area of a centralizing ring whereby the ring forms a line contact with the inner surface of a surrounding tubular body.
An important object of the present invention is to provide a centralizing ring that may be employed in the vertical assembly of concentric tubular components whereby the radially outer surface of such centralizing ring engages the internal surface of the surrounding tubular and is configured to freely advance axially along such surface.
An important object of the present invention is to provide a lifting device that permits a tubular body equipped with annular centralizing rings to be lifted from a horizontal orientation, vertically oriented in a position coaxially with a larger tubular within which is to be received, lowered centrally through the larger tubular, and positioned axially at the correct axial location relative to the surrounding tubular. A related object of the present invention is to provide an annular shoulder and radial end surfaces on the lifting device whereby the lower end of the smaller tubular rests on the annular shoulder while the radial end surfaces centralize the smaller tubular as it is lowered through the larger tubular.
An object of the present invention is to provide a lifting device that can support an outer tubular body and an inner tubular body and automatically position the two bodies axially relative to each other while the bodies are vertically oriented.
An object of the present invention is to provide a single assembly device for lowering a vertically oriented tubular through a larger surrounding tubular and subsequently raising both tubulars with a single lifting movement of the assembly device.
The foregoing, as well as other, objects, features, and advantages of the present invention will become more readily apparent and better understood from the following drawings, specification, and claims.